1. Field of the Invention
The present invention relates to a zoom lens system and a camera including the same.
2. Description of the Related Art
In an image pickup optical system (a shooting optical system) used in a camera such as a TV camera, a digital still camera, and a video camera, a small and lightweight zoom lens system which has a large aperture, a wide field angle, a high zoom ratio, and high optical performance over an entire zoom range and an entire object distance range is demanded.
In addition, a zoom lens system having an image stabilization function of reducing (compensating for) shot image blurring caused from a shake when shooting by a camera carried on a shoulder or a vibration when shooting by a camera mounted on a vehicle or the like is demanded.
Conventionally, there is known a zoom lens system having the image stabilization function of compensating for the shot image blurring. A variable apex angle prism having a variable prism apex angle is disposed in an optical path of the zoom lens system. When the zoom lens system oscillates, the prism apex angle is changed according to the oscillation for compensating for the shot image blurring (see Japanese Patent Application Laid-Open No. S61-223819 (corresponding to U.S. Pat. No. 4,927,250), Japanese Patent Application Laid-Open No. S11-101941, Japanese Patent Application Laid-Open No. H07-104218 (corresponding to U.S. Pat. No. 5,677,792), and Japanese Patent Application Laid-Open No. H08-015648).
In general, if the method of changing the prism apex angle of the variable apex angle prism for compensating for the shot image blurring is used, a decentering aberration occurs in the zoom lens system by the variation of the prism apex angle when the image stabilization function is activated. Therefore, image quality is apt to deteriorate when the image stabilization function is activated.
There is known a paper about the decentering aberration caused by the image stabilization function, which is handled based on an aberration theory (Optical Society of Japan, Nineteenth Optical Symposium Proceedings, Lecture No. 17).
In general, the method of changing the prism apex angle of the variable apex angle prism for compensating for the shot image blurring has a merit that the shot image blurring can be compensated relatively quickly and easily even if the vibration and the shot image blurring are large.
However, there is a tendency that a decentering aberration may frequently occur corresponding to the variation of the prism apex angle of the variable apex angle prism, and hence the image quality deteriorates significantly. Therefore, it is important to set appropriately a structure of the variable apex angle prism and a position of the variable apex angle prism in the optical path, in particular, structure of lens units disposed before and after the variable apex angle prism, in order that the image stabilization is performed quickly and that the decentering aberration generated in the image stabilization process is reduced.
If those structures are not appropriate, a decentering aberration may frequently occur when the image stabilization function is activated, and hence image quality deteriorates significantly when the image stabilization function is activated.